1. Field of the Invention
The present invention relates to recording control apparatus having actuators as recording elements on a recording head and capable of forming characters and symbols on a recording medium by controlling the voltage supplied to the actuators.
2. Description of the Prior Art
The recording head employed in thermal printers or ink jet printers is generally provided with an array, in a matrix or line form, of actuators as recording elements to which voltage is selectively supplied to form characters and symbols on a recording medium. However such actuators are inevitably associated with fluctuations in the characteristics thereof among themselves and between different heads. For example resistors used in the thermal heads are prepared by semiconductor technology such as diffusion, thin film formation or thick film formation but the fluctuation in resistance is unavoidable in the technology.
Such fluctuation in resistance leads to a density fluctuation in printing on recording paper due to the difference in heat generation among the resistors, thus resulting in a deteriorated print quality.
It is empirically confirmed that the difference in density of characters printed on recording paper is visually not noticeable if the fluctuation in resistance is within a range of .+-.5% with respect to the mean value thereof. In the case of thermal heads having resistors arranged in a 5.times.7 or 1.times.7 dot matrix, it is technically possible to maintain the fluctuation in resistance within the range of .+-.5% with respect to the mean value within the same head.
Furthermore it is generally considered economically advantageous to maintain the tolerance in the fluctuation of resistance between different heads within a range of .+-.15% with respect to the means resistance. Consequently, the conventional method of avoiding the fluctuation in density of printed characters among different heads consists of classifying the heads according to the average resistance into the following three groups:
group A: 15%-5% of the mean resistance, PA1 group B: 5%--5% of the mean resistance, and PA1 group C: -5%--15% of the mean resistance,
and changing the supplied voltage according to the groups thereby achieving constant heat generation.
FIG. 1 shows an example of circuitry for such a case, in which a slide switch 1 is connected at a terminal A directly, at a terminal B through a diode D1, and at a terminal C through diodes D1 and D2, to a power supply +V. Also a common terminal D of the slide switch 1 is connected to the common electrode for resistors R1, R2, . . . , Rn of a thermal head 2. Those resistors are connected at the other terminals thereof to drive transistors Tr1, . . . , Trn.
In the above-mentioned circuit the heat generation by the resistors is achieved by selective switching of drive transistors Tr1-Trn.
In the above-explained circuit, three different voltages can be supplied to the common terminal, depending upon the voltage drop across the diodes D1, D2 according to three different positions A-D, B-D and C-D of the slide switch. Positions A-D, B-D and C-D are selected respectively when the average resistance of the thermal head 2 belongs to the group A, B or C. In this manner a constant heat generation can be obtained through appropriate selection of the diodes D1, D2, and it is rendered possible to avoid fluctuation in the print density on a thermographic paper sheet. Such a conventional method is however defective in that the slide switch 1 has to be suitably shifted each time the thermal head 2 is exchanged, and in that the switch position A-D providing a high voltage, if erroneously selected for a low-resistance head of group C, will not only provide an abnormally high print density but also may undesirably affect the reliability of the recording head.